Figure 3-24. The winter constellation Orion, the celestial hunter, with his belt and sword. Even with a small telescope you can see the Orion Nebula, which is a place 1,500 ly away were new stars are currently forming. (NASA/STScI)
Figure 3-25. A close-up of the Orion Nebula made by combining images from NASA’s Hubble and Spitzer Space Telescopes. It is estimated that this cloud contains 1,000 stars which are in the process of formation. Swirls of green in Hubble’s ultraviolet and visible-light view reveal hydrogen and sulfur gases that have been heated and ionized by intense ultraviolet radiation from the Trapezium’s stars. The Trapezium stars are the yellow smudge near the center of the image. Meanwhile, Spitzer’s infrared view exposes carbon-rich molecules. These organic carbon molecules have been illuminated by the Trapezium’s stars, and are shown in the composite as wisps of red and orange. (NASA/JPL-Caltech/STScI) At this point in its evolution, the Sun had not yet started its thermonuclear fusion. It is thought to have been a type of star called a “T Tauri star.” Within 50 million years, the temperature and pressure at the core of the Sun became so great that its hydrogen began to fuse, creating an internal source of energy that countered gravitational contraction until hydrostatic equilibrium was achieved. “Hydrostatic equilibrium” means that the inward pull of self-gravity is balanced by the outward push of the gas pressure. As a result, the Sun stabilized. It stopped to contract. Thermonuclear fusion marked the Sun’s entry into the prime phase of its life, known as the main sequence. “Main sequence” stars derive energy from the fusion of hydrogen into helium in their cores. The Sun remains a main sequence star today. 63